The invention relates to a process and apparatus for the fractionation of engine fuel on board a motor vehicle.
In motor vehicles having an internal-combustion engine, specific fractions of commercially available engine fuel are required for special uses. It is known, for example, that in vehicles with Otto engines the use of low-boiling fuel fractions in the cold-start phase lowers the hydrocarbon emissions.
It is an object of the invention to provide a process for separating a low-boiling fuel fraction from an engine fuel on board a motor vehicle, which requires only a small space and is lightweight. Furthermore, the required low-boiling fuel fraction must be available as early as at the start of the vehicle.
This object is achieved by the process according the invention, in which the fractionation of the fuel takes place by means of a gas delivery pump by vacuum evaporation with a subsequent pressure condensation. The process can be carried out at ambient temperatures, so that no additional feeding or removal of heat is required.
The separated low-boiling fuel fraction is particularly suitable for use as engine fuel which lowers the hydrocarbon emissions during the cold start phase of the engine.
Furthermore, the separated low-boiling fuel fraction can be utilized as a reducing agent for nitrogen oxides removal catalysts in the lean engine exhaust gas.
The fractionation of the fuel can take place from the fuel supply tank of the motor vehicle or from an additionally existing intermediate tank. In the latter case, the higher-boiling fraction remaining after the separation can be used as engine fuel in engine operating phases suitable for this purpose.
Because of the above-mentioned advantages, the process according to the invention is suitable for use in all mobile systems, such as passenger cars and utility vehicles.
The equipment-related expenditures for carrying out the process are low. Moreover, complex components, such as evaporators, are not required, so that the space and the weight can be kept low.
Lowest-boiling compounds, such as butane, which can be condensed during distillation only with very substantial cooling (&lt;0.degree. C.), can be obtained by the process according to the invention at comparatively low expenditures.
Another important advantage of the process according to the invention lies in the fact that, because of the electric power available in the motor vehicle, the desired fuel fraction can be provided for the operation of the gas delivery pump immediately at the start of the engine. Thus, a supply tank for this fuel fraction is not necessary.
By constantly suctioning-off the gas phase from the fuel tank, evaporation losses are largely avoided. As a result, the hydrocarbon emissions from the fuel tank are lowered and the efficiency of the motor vehicle is increased.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.